Plastic materials are lightweight, highly tough and easy to be dyed, and therefore are widely used recently for various types of optical materials, particularly eyeglass lenses. Optical materials, particularly eyeglass lenses, are specifically required to have, as physical properties, low specific gravity, high transparency and low yellowness, high heat resistance, high strength and the like, and as optical properties, high refractive index and high Abbe number. A high refractive index allows a lens to be thinner, and a high Abbe number reduces the chromatic aberration of a lens. However, as the refractive index is increased, the Abbe number is decreased. Therefore, it has been studied to improve both of the refractive index and the Abbe number. Among methods which have been proposed, the most representative method is a method using an episulfide compound as described in Patent Document 1.
Moreover, it has been studied to achieve a high refractive index, and compositions consisting of sulfur, an episulfide compound and thiol described in Patent Documents 2 and 3 have been proposed.
Furthermore, optical materials obtained by introducing thiourethane in a polyepisulfide compound in order to improve the strength have been reported (Patent Documents 4 and 5).
However, when thiourethane is introduced, it causes reduction in heat resistance, generation of odor at the time of cutting work and occurrence of uneven polymerization called striae, and for this reason, techniques in which a composition ratio is limited or a viscosity is limited have been reported (Patent Documents 6-8).
However, white turbidity may occur in these compositions containing an episulfide compound when polymerized and cured. Since compositions are used for optical materials, when white turbidity occurs after curing, all becomes defective products, resulting in heavy losses. Accordingly, a technique which makes it possible to predict whether or not white turbidity will occur after curing and to determine quality at a stage before curing has been desired.